When is 3x80 amps 25kVA?

Thanks for the interest in my earlier question.

I have just paid my DNO a lot of money to install a 3-phase connection with a maximum load of 25kVA.

Is my physics wrong when it tells me that when they fitted 80 amp fuses at the incomer I have 25kVA? I think it's wrong to multiply 230(v) x 80 x 3, but is it correct to multiply 415v x 80 which = 33.2kVA? And if 100 amp fuses were installed do I have 41.5kVA?

Back to shool I think.

25kva at 3 phase 400v is about 36A per phase

3 X 80 amps @230v is about 55kVA

looks more like 18KVA per phase, so 54KVA total. oops, above post said it first.

KVA = VL x IL x 1.732

400 x 80 x 1.732 = 55.4 KVA

Just because the DNO has fitted fuses that can handle more than 25KVA, don't just load up and go. You may end up with a sternly worded bill including a charge for exceeding your allowance.

I think it's wrong to multiply 230(v) x 80 x 3,

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No, it's correct. When dealing with 3-phase supplies that may or may not have a neutral, it's customary to always calculate from the line-line voltage (normally 400V) in which case P = I * 400 * sqrt(3) as this is always applicable. But the power available is the same whether it is drawn by loads wired between the lines at 400V, or from line to neutral at 230V, hence P = I * 230 * 3.

AndyRob said:

I have just paid my DNO a lot of money to install a 3-phase connection with a maximum load of 25kVA.

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Working backwards, if you had a 3ph load of 25kVA pf 0.8, 400v, line current of say 45A would you use a 80A for the circuit fuse?

Maybe the DNO have made a simple typing error and should read 55kVA and not 25?

DefyG said:

Working backwards, if you had a 3ph load of 25kVA pf 0.8, 400v, line current of say 45A would you use a 80A for the circuit fuse?

Maybe the DNO have made a simple typing error and should read 55kVA and not 25?

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I suspect that a single cable size and 80A fuses are just the standard install package the DNO puts everywhere. As long as the cable is properly protected, then perhaps you can pull more load than you officially should, depending on how granular the feedback from the meter is.

ElectroChem said:

I suspect that a single cable size and 80A fuses are just the standard install package the DNO puts everywhere. As long as the cable is properly protected, then perhaps you can pull more load than you officially should, depending on how granular the feedback from the meter is.

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OP here again.

Thanks for all the replies - I've now had a long chat with a very helpful chap at my DNO - you might be interested in what was said.

First, whilst I originally applied for 60kVA, that number was a shot in the dark as I had no idea what the cost would be so I thought of a number and doubled it (sort of). As it happens, for any application over 25kVA there's a hefty charge for the quotation itself so I changed the application to the lower number for which there is no charge for a quotation . I only wanted to add a heat pump and EV charge point to a domestic property -it wasn't for a Bit-Mining computer room or whatever.

So, as Electrochem said, the DNO has standard waveform underground cables which they use for "normal" jobs. I actually have another two 3 phase supplies feeding different buildings (so three in total) and have sole use of an 11KV transformer feeding a 100m long, 50mm o/d, aluminium three core cable. That splits into 32mm o/d cables going into the separate buildings. The now standard fitment (just about everywhere, I'm told) 80 amp Lawson fuses are at the end of every cable - even if the rating on the outside of the carrier says 100A - that's the rating of the fuse carrier, apparantly.

So, though I'd applied and paid for 25kVA (about 31kw using the 0.8 correction for real v apparent power) on my most recent connection I could potentially draw much more than that - a total of around 165kVA - over all three supplies.

The transformer, pictured below, shows a rating of 66.7 amps at 433v. With the numbers you've all quoted above I was concerned about unwittingly overloading my part of 'the grid' - hence the phone call to the DNO.

Whatever the potential overload actually is, the DNO works on several assumptions. First that not all potential loads are in use at anything like their maximum 80A per phase at the same time. Second, if they were, that the slow-blow 80A fuses and/or those at the transformer, would not blow at anything like their rating - it could be many (many, many) minutes at a much larger current before they would - I was told. And the transformer can handle much more than its rated maximum - even for some hours.

The moral? Rules (numbers in this case) are for fools and the guidance of wise men? I'm not sure whether I'm overly happy with that but, in my situation, if it all works in practice then so be it.

To widen the conversation, in talking to the DNO people installing my 3-phase supply, I think we have a time-bomb waiting to go off in our local electricity infrastructure. In 40 months time no new houses can have gas or oil heating - so, presumably, heat pumps for them plus all the other homes where people want to switch to electric heating. And in less than 9 years you won't be able to buy a diesel or petrol car - only hybrid or all-electric. And only all-electric cars in 14 years time. How many homes, streets, estates, towns have the supply capacity to accommodate this demand? None around me.

You worry in 40months time no new houses will have gas, you must also realise there will be a drive for them to have alternative sources like solar, they will have significantly better energy efficiency compared to the existing old stock housing out there and may even have some form or air source or ground source heating, recently done a few myself and surprisingly efficient even with snow piled up outside.

I also believe people will learn to put on a jumper instead of having their thermostat on full whack most of the year.

Darkwood said:

You worry in 40months time no new houses will have gas, you must also realise there will be a drive for them to have alternative sources like solar, they will have significantly better energy efficiency compared to the existing old stock housing out there and may even have some form or air source or ground source heating, recently done a few myself and surprisingly efficient even with snow piled up outside.

I also believe people will learn to put on a jumper instead of having their thermostat on full whack most of the year.

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I agree entirely. The problem is that, for homes, wind power isn't a viable solution that compares with solar and, for new solar generation, the domestic market supply chain has largely collapsed because of the removal of the FIT susbsidy.

A Building Reg's requirement could be used to ensure new-builds, probably on an individual but also an 'estate' basis, included a certain amount of on-site generation. Schools built in the 2000's under the BSF (Building Schools for the Future) had generous incentives to install combined heating and electric power generating systems (CHP) using renewable fuels. And thousands of schools took advantage of their large roof areas to install solar panels.

But your last point is especially important - my children's headteacher had a Wooli-Pulli policy which basically meant that kids - and staff - couldn't wear just thin T-shirts or vest tops and then complain of being cold. A long time ago didn't Margaret Thatcher introduce a maximum office temperature of 18c that practically no-one observed?